energy and allowing the body to move against the force of gravity. Muscles must then dissipate the mechanical energy produced to accelerate the body during landing to decelerate the body. Therefore, a simple hop requires muscles to act as both the motors and dampers of the body. We use landing studies in toads to examine how motor control strategies stabilize the body during landing. We also aim to understand the specific properties of muscles that allow them to be effective dampers and dissipate energy without damage or injury. See publications #28,#25, and #24 for

more detail. This work was supported by NSF IOS grant #1051591.

Muscle's automatic transmission

Compliant muscles

of jumping frogs

The remarkable jumping ability of frogs has long

captured the imagination of writers, natural

historians and functional biologists. In fact,

frogs jump much further than they should given the amount muscle present in their legs. Its been shown that the mechanical power required for a frog jump far exceeds the maximum mechanical power that can be produced by all of their leg muscles combined. Therefore, the frog jump is a great example of an organism circumventing the limits of muscle’s contractile machinery to perform a spectacular locomotor task. Our work has shown that the passive elastic properties of frog leg muscles play an important role in improving force production during jumps (#15). Compared to mammals frog leg muscles are more compliant allowing them to operate over long excursions while maintaining high force production. We hypothesize that passive muscle properties may relate more broadly to both the postural and functional differences observed in vertebrate locomotion (#25).

Function & mechanics of Aponeuroses

The elastic structures of many muscles include both an extramuscular “free tendon” as well as a sheet-like aponeurosis. An important distinguishing feature of aponeuroses is that these planar tendinous structures function as the attachment and insertion surfaces of muscle fascicles and therefore surround a substantial portion of the muscle belly. As a result, aponeuroses must expand both parallel (longitudinal) and perpendicular (transverse) to a muscle’s line of action when contracting muscles bulge to maintain a constant volume. Therefore unlike free tendons, aponeuroses are loaded biaxially during muscle contraction. We have studied the morphology, the material properties (#12, #13), and the in situ strain patterns of aponeuroses to understand the functional implication of biaxial loading in these tendon sheets (#12, #13, #33).

The image of a flexing bodybuilder is a familiar reminder that muscles change shape as they contract. It turns out that this bulging of muscles has an important function. Just by bulging differently, muscles can change the “gear” through which they power movement. Moreover, muscles can shift gears simply in response to the load applied to them without input from the nervous system. Therefore, variable gearing in muscle is similar to a car’s automatic transmission system, allowing muscles to shift from a low gear during forceful movements to a high gear during fast movements (#10).This work has revealed a novel mechanism for modulating how the forces and displacements produced by a muscle’s contractile machinery can be effectively used during mechanically diverse behaviors. More recently we have extended this work to show that operating with a high gear can protect muscles during a rapid stretch (#23). Finally, the mechanism revealed by this work has been used to inspire the design of actuator arrays (#21).

Overview

Most of the basic features of muscles have remained unchanged across vertebrate evolution. As a result, all muscles operate with a similar set of intrinsic physiological constraints, which must ultimately define the boundaries to the speed, force and cost of movement. Research in my lab aims to understand how organisms overcome these constraints to meet the diverse mechanical demands of movement. In my lab we use an integrative approach to reveal how interactions between muscle physiology, muscle-tendon structure, and biomechanics ultimately determine locomotor performance.

Elastic Mechanisms

Most muscles operate in series with tendons, which function as efficient biological springs to store and release strain energy. Tendons can decouple length changes of contracting muscle from the motion of the joint. As a result tendons have the potential to minimize the energetic cost of locomotion, amplify muscle power, and protect muscles during locomotion (#17). Specifically we have focused on how tendons can reduce the rate of energy input to muscles during activities that require muscles to dissipate mechanical energy (#16).

Muscle ECM

Muscle fibers and fascicles are embedded in collagenous scaffolding which contribute to muscle’s passive stiffness, function to provide structural integrity and transmit forces. Muscle ECM has been shown to become fibrotic due to atrophy, aging or neuromuscular disorders and our group is examining how changes in mechanical properties of the ECM affect physiological function (#34,35). We are primarily using an established aging model system to address basic questions regarding muscle-collagen interactions. This work is funded by NSF CMMI grant #1436476.